Method and system for producing glass, in which chemical reduction of glass components is avoided

ABSTRACT

In the apparatus for producing glass reduction of reduction-sensitive components in the glass melt is reduced or preferably is prevented during the melting and/or fining processes by introducing an oxidizing agent into the glass melt. The apparatus has a melt crucible, a fining vessel, and a device for conducting oxygen and/or ozone into the glass melt in the melt crucible and/or fining vessel, in order to suppress reduction of reduction-sensitive components of the glass melt. A preferred embodiment of the apparatus has a metallic skull crucible, which includes the melt crucible and/or the fining vessel. The apparatus preferably includes a homogenization unit connected to the fining vessel to receive glass melt from the fining vessel in order to further process the glass melt after refining.

CROSS-REFERENCE

This is a divisional, filed under 35 U.S.C. 120, of U.S. patentapplication Ser. No. 11/835,689, which was filed Aug. 8, 2007 in theU.S.A. The subject matter of this divisional application is alsodisclosed, at least in part, in German Patent Application DE 10 2006 037828.8, which was filed in Germany on Aug. 12, 2006, and in DE 10 2007008 299.3-45, which was filed in Germany on Feb. 16, 2007. The subjectmatter of the foregoing German Patent Applications, which provide thebasis for a claim of priority of invention under 35 U.S.C. 119 (a) to(d), is hereby incorporated herein by explicit reference thereto.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to a method and an apparatus for producingglass, in which the reduction of reduction-sensitive components of theglass is decreased and preferably is avoided during the melting andfining process. The glass is preferably glass with a high refractiveindex. According to the invention the term “reduction-sensitive” meanssensitivity to both reduction and oxidation reactions, i.e.“reduction-sensitive” also means “redox-sensitive”.

2. The Description of the Related Art

Many high-index materials and glass, especially those used for optical,fiber-optic and display-applications as well as for applications forprotection and passivation of electronic parts and components, arecomposed so that the melt technical production leads to a considerableloss of performance in conventional systems and facilities concerningvery important properties for the particular application such astransmission, refractive index position, uniformity, electricalresistance and compressibility, thus rendering the productionuneconomical.

Such materials and glass compositions comprise components that arereduction sensitive and/or corrosive in the molten state. Reductionsensitive, so-called polyvalent, components can have different redoxstates (oxidation states) in the melt. The equation for the redoxequilibrium of such a component is:

wherein M^((x+n)+) is the oxidized form and M^(x+) the reduced form ofthe species M. The redox partners are usually oxygen ions (O²⁻) presentin the melt and oxygen dissolved in the melt (O₂).

For this redox equilibrium and under the precondition that the oxygenanion concentration is constant (O²⁻=const.) the equilibrium constant Kis given by equation (1):K=([M ^(x+)]·[O₂]^(n/4)]/([M ^((x+n)+)])  (1).

From equation (1) and equation (2), ΔH−T*ΔS=−RT*ln K, the followingdependency of the redox equilibrium concentration ratio[M^((x+n)+)]/[M^(x+)], [Ox]/[Red], respectively, on the temperature Tand on the oxygen concentration [O₂] results:ln([M ^((x+n)+) ]/[M ^(x+)])=ΔH/(R·T)−ΔS/R+(n/4)·ln [O₂]  (3)wherein ΔH=enthalpy of the reaction, ΔS=entropy of the reaction,R=specific gas constant.

The outcome of this is that the redox equilibrium is shifted towards thereduced species M^(x+), if the temperature T rises and/or the oxygenconcentration [O₂] decreases.

With decreasing temperature T and/or rising oxygen concentration [O₂]the redox equilibrium is shifted towards the oxidized speciesM^((x+n)+).

The redox relationship of the oxidized form and reduced form of acomponent at a distinct temperature and a distinct oxygen concentrationis finally determined by the composition of the melt, the substance andmatrix specific thermodynamic variables (ΔH and ΔS) and the possibleredox reactions with other polyvalent components. For example in a meltof the composition (% by weight): 8.8% Na₂O; 29.6% SrO; 61.1% P₂O₅ and0.5% SnO₂ at 1200° C. and with an oxygen partial pressure of 0.21 bar(this is the partial pressure in the atmosphere) about 94% of the tinare present in the form of Sn⁴⁺ (oxidized form), whereas only 6% arepresent in the form of Sn²⁺ (reduced form). If the temperature isincreased to 1500° C. (at unchanged oxygen concentration i.e. unchangedpartial pressure), the redox relationship is changed. In that case thethermodynamic equilibrium shifts so that 47% of the tin are present asSn⁴⁺ (oxidized form), 50% are present as Sn²⁺ (reduced form) and already3% are present as elemental metallic tin. If the oxygen concentration iselevated i.e. the partial pressure is increased to 1 bar at 1500° C.,57.5% of the tin are present as Sn⁺ (oxidized form), 41% are present asSn²⁺ (reduced form) and only 1.5% are present as elemental metallic tin.The phosphate ions in this melt underlie the thermodynamic redoxequilibrium, too. At 1200° C. and an oxygen partial pressure of 0.21 bar(this is the partial pressure in the atmosphere) about 99.9% of thephosphorus are present as P⁵⁺ (oxidized form) and only 0.1% as P³⁺(reduced form). At a temperature of 1500° C. and reducing conditions,for example at an oxygen partial pressure of 10⁻⁵ bar, about 89% of thephosphorous are present in the form of P⁵⁺ (oxidized form), but already11% are present as P³⁺ (reduced form) and 0.1% are even present aselemental phosphorous (source: “Das Redoxverhalten polyvalenter Elementein Phosphatschmelzen und Phosphatgläsern”, Dissertation AnnegretMatthai, Jena 1999).

Critical for the product properties of materials to be produced are, inconnection with the reduction of the components in the materials, adirect effect (decrease) on the optical transmission values due to thereduced species themselves on the one hand and an indirect effect(decrease) on the optical transmission values due to reaction of reducedspecies with container materials. Furthermore important properties ofthe materials, such as electrical resistance and the dielectricstrength, are influenced negatively, but the reduced species or theircorrosion products also influence the crystallization and moldingproperties.

The reducible species will directly influence the transmissionproperties if these materials are not present in their highest possibleoxidation state. High oxidation states normally have electronconfigurations that forbid electron transitions due to absorption oflight in the visible spectral region, which influences the opticaltransmission of the material. But in case these components are presentin lower oxidation states, electron configurations may occur that allowelectron transitions. These lead to absorption of light in the visiblespectral region and, thus, to discoloration. Such so-called polyvalentcomponents are, for example: niobium, phosphorous, vanadium, titanium,tin, molybdenum, tungsten, lead and bismuth.

If these components are further reduced thermally or chemically, theycan have an oxidation state of 0 and, hence, be present in elementalform. Precipitation of particles and/or crystals in the nanometer rangeoccurs. This leads under the influence of light to diffraction andscattering effects in the material that influence the transmission inthe visible spectral range, too. But other properties like theelectrical resistance, the dielectric strength and the crystallizationproperties can also be influenced.

If the precipitated particles or crystals grow, tension and defectsoccur in the material that can during irradiation with highenergy-densities (for example: lasers) lead to destruction of the glass.As described in DE 101 38 109 A1 such particles must be oxidized againthrough elaborate processes, for example using highly toxic gaseouschlorine, in order to ameliorate the optical properties of the glassafter the melting process. The addition of nitrates in the glass batchthat provides for strongly oxidizing conditions in the melt byliberation of NO₂ and other nitrous gases has to be rejected based onenvironmental and working security grounds. The described process isalso highly dangerous in connection with free phosphate (P₂O₅), becauseit can lead to explosive reactions.

Components that can thermally and/or chemically be reduced to theelemental state in the melt are for example phosphorous, tin, germanium,lead, arsenic, antimony, molybdenum, bismuth, silver, copper, platinummetals and gold.

If there is an affinity or a tendency towards alloy formation betweenthe components reduced in the melt and the container material, thereduced components alloy with the container material and are thuscontinuously extracted from the melt by chemical equilibrium, theformation of which would lead to an abating of the reaction. So a cycleis set up that in the end leads to a destruction of the crucible,because of the alloy formation the resistance and the melting point ofthe crucible materials is strongly decreased. This is especiallycritical in case of crucibles of the platinum group. For example, thealloying of 5% phosphorous with platinum leads to a decrease in meltingpoint from 1770° C. to 588° C. with the resulting effects on thedurability of the crucible.

In a less dramatic case the in situ formed alloy is at once dissolved inthe melt and a large amount of crucible material is introduced into themelt occurs. In the case of platinum elements this is connected with adiscoloration and a worsening of the transmission properties.

It is especially critical in order to achieve high refractive indexes ofn_(d)>1.7, preferably n_(d)>1.7 5, and/or minimum possible softeningtemperatures that are of great importance for precision and precisepressing, that high amounts of reducible compounds are introduced intothe materials and glasses.

The use of so-called flameproof materials having an oxidic oroxidic-ceramic basis, for example, zirconium, silicate, or aluminiumoxide material, only solves a part of the above-described problems andis additionally not an economically reasonable solution, either. Thesematerials are indeed not reducing, do not show any alloy formingtendencies towards elemental precipitations and are relatively stabletoward many melt compositions as far as corrosion and dwell time areconcerned. But when they are attacked by the melt they dissolve in partand are “bad-natured”, that is these flameproof materials can lead tofaults in the glass.

Especially aggressive attack of high-index melt compositions that shouldadditionally be workable by precise pressing is not acceptable, becausethe dissolution of the crucible and the entry of the material into themelt lead to unwanted changes in properties of the materials andglasses, especially an increase in the transformation temperature,changes in viscosity properties, changes in refractive index and Abbenumber as well as changes in transmission. Furthermore areas are formedthat are enriched with the flameproof material, which become visiblebecause of striae and refractive index changes in the material.

As a further effect of the strong aggressive attack on flameproofmaterials, apart from the considerable worsening of properties anduniformity of the glass, an in part extreme shortening of the dwell timeof the melt equipment arises causing extensive costs. On the one handcosts arise because of the need to renew the melt unit and on the otherhand because of repeated downtime costs.

The continuous melting and fining of corrosive materials and glass insystems with cooled walls on which the material freezes and forms acontact area of specific material is well known for many technical andoptical glasses and patented, too (DE 102 44 807 A1, DE 199 39 779 A1,DE 101 33 469 A1). The container, in which the melt is heated and inwhich the fining process takes place, usually comprises meander-shapedcooling circuits and usually high-frequency radiation is used for theheating of the melt. The forming boundary layer of specific material toa large extent prevents the attack of the melt on the wall material.Hence no contamination of the melt from wall material takes place. Allthese inventions claim, inter alia, the melting and in part the finingof corrosive, optical glasses of high purity in these so-calledskull-devices. Because of the high reduction potential and thecomparatively high temperatures that are needed for injection andmelting of high-index melts, especially in the niobium oxide/phosphorousoxide system, none of the cited documents offers the potential toproduce the claimed glasses in the necessary quality and with thenecessary properties.

SUMMARY OF THE INVENTION

It is an object of the present invention is to provide an apparatus forperforming a process for producing glass, in which the reduction ofreduction-sensitive components of the glass is decreased and preferablyavoided. Especially the glass produced in the apparatus has a highrefractive index, for example heavy metal-phosphate glasses, heavymetal-borate glasses or heavy metal-silicate glasses.

According to the present invention this is achieved by introducing orinjecting oxidizing agents into the fining vessel and preferably intothe melt crucible, too. Preferably the oxidizing agents are oxygenand/or ozone. Accordingly the apparatus is provided with a means forconducting the oxidizing agents into the fining vessel and optionallyinto the melt crucible.

In the process performed in the apparatus according to the presentinvention preferably melting as well as fining processes take place inmelt vessels that are cooled using cooling agents.

Surprisingly it has been discovered that the melting process ofreduction-sensitive, high-index materials and glass is well controllableat the high temperatures needed for the process, if the oxidation stateof the melt is held within the strongly oxidizing range through apermanent increase of the oxygen concentration from the beginning andthe melt itself has no contact with the vessel walls of the meltcrucible. This is achieved by cooling the vessel walls with a coolingagent, so that a solidified crust (skull) of specific material isformed. Because of the use of oxygen and/or ozone for bubbling duringthe whole process, it is possible to avoid using highly toxic oxidizingagents such as chlorine, fluorine or nitrogen dioxide from appliednitrates.

In the case of reduction-sensitive glass bubbling with oxygen and/orozone has also proven helpful in the conditioning zone, in order tofurther improve the transmission values.

Energy is introduced into the melt according to its electricalresistance either directly conductively via electrodes or directlyinductively via high-frequency radiation.

In the case of heating the melt with electrodes, the electrodes must becooled intensively, in order to minimize the aggressive attack of themelt on the electrodes. Highly conductive, corrosion-resistant andnot-reducing materials like tin oxide, gold or platinum metals(especially platinum and iridium) are used as the electrode material.

In the case of heating with high-frequency radiation the melt cruciblesthat are cooled by cooling agents must be constructed such that they arepermeable to high-frequency electromagnetic alternating fields.

The threshold between these two possible heating methods occurs at aspecific electrical resistance value ρ of the melt of ca. 10 Ω·cm at thecorrespondingly necessary process temperature. This value can to someextent vary according to the respective system and circumstances and,hence, is more a threshold range than a fixed threshold.

The melt crucible preferably consists of (in case of high-frequencyheating usually slit) walls that are cooled by a cooling agent and a (incase of high-frequency heating usually slit) bottom also cooled by acooling agent. A material having high thermal conductivity is used forthe walls. Preferably metals or metal alloys are used for that purpose.These can be coated or uncoated. A preferred embodiment of the cruciblewalls consists of an aluminium alloy. Further embodiments of thecrucible walls consist of nickel-based alloys, copper, brass, noblemetals or high quality steels. Coatings can consist offluorine-containing synthetic material or of different metals.

Preferably the melt is heated with a high-frequency electromagneticalternating field directly inductively during operation. The conductiveelectrical heating via electrodes is only used, if the electricalconductivity at the highest acceptable melting temperature is notsufficient for a direct high-frequency heating of the melt. A heating ofthe melt with radiant heaters, electrically or in the form of a burnerrunning with fossil fuels, is also possible. For the starting process(for example the injection of the crucible content into theelectromagnetic alternating field) an additional heater is preferablyused in the form of a fossil fuel burner.

The oxygen partial pressure is maintained by bubbling oxygen and/orozone through the crucible bottom and, thus, highly oxidizing conditionsare set up throughout the entire melt volume. The bubbling gas can beinput to the melt via conventional bubbling nozzles arranged at specificlocations in the melt vessel or by foamed, porous or perforated cooledstructures that provide a laminar flow of bubbling gas. These conditionsprevent on the one hand the reduction of individual melt components,especially of phosphates or of P₂O₅ and of polyvalent heavy metal oxidesin lower valence states, color-imparting or even metalliccolor-imparting and alloying species. On the other hand oxidation of allpolyvalent species occurs (also the fining agents). Because of theformation of the specific skull-crust aggressive attack on the meltcrucible is prevented and, hence, no crucible material is introducedinto the melt. Especially the content of highly corrosive P₂O₅ in theglass batch can thus be nearly arbitrarily large. The advantage of(so-called “free”) P₂O₅ lies in the high purities achievable therewith.In contrast to highly pure so-called “free” phosphate (P₂O₅) “bound”phosphates always show a high degree of impurities because of theirproduction process.

The melt is afterwards transferred from the melt crucible into a finingvessel.

In a special embodiment this can be done by a directly heated connectionpipe made of high quality steel (for example if the melt shall becompletely free of silicate) that has a minimum possible length (notlonger than 500 mm) or by an indirectly heated connection pipe made ofsilica glass or ceramic (for example if the melt shall be completelyfree of noble metals). The advantages of this system are the thermaland/or fluidic decoupling of the melt volume and fining volume, theoutstanding controllability and the temperature inspection of the meltstream.

In another embodiment the melt is transferred from the melt crucibleinto the fining vessel via a short (not longer than 300 mm)—in theactual operation un-cooled—segment with cooled walls. In order to startthe process the melt in this segment is heated with a radiant heater (inthe form of a fossil fuel burner or electrically). The advantage of thissystem is the complete separation of the whole high-temperature part ofthe system from the components underlying corrosion, and hence thenearly absolute prevention of the entry of foreign matter into the melt.

The fining vessel like the melt crucible preferably consists of (in caseof high-frequency heating usually slit) walls and a bottom that arecooled by a cooling agent. These likewise preferably consist of metal ora metal alloy. It can be coated or uncoated. A preferred embodiment ofthe vessel walls consists of an aluminium alloy. Further embodiments arehowever vessels made of nickel-based alloys, copper, brass, noble metalsor steels. Coatings can consist of fluorine-containing syntheticmaterials or other materials. Preferably the melt is heated directlyinductively via a high-frequency electromagnetic field during operation.A conductive electrical heating via electrodes is also possible, but isonly applied, if the electrical conductivity at the maximum applicablemelting temperature is not sufficient for a direct high-frequencyheating of the melt. The heating by radiant heaters, electrical devicesor a burner that operates with fossil fuel is also possible. Anadditional heater in the form of a fossil fuel burner is preferably usedfor the starting process of introduction of the crucible contents intothe electromagnetic alternating field.

The oxygen partial pressure is maintained by bubbling oxygen and/orozone through the crucible bottom and, thus, highly oxidizing conditionsare adjusted throughout the entire melt volume. The bubbling gas can beinput to the melt via conventional bubbling nozzles arranged at specificlocations in the melt vessel or by foamed, porous or perforated cooledstructures that provide a laminar flow of bubbling gas.

This second bubbling step brings the oxygen partial pressure in the meltback to the starting level adjusted in the melt crucible. So, reductionprocesses are effectively avoided, during the melting process reducedspecies are oxidized again and a redox buffer for the fining process isestablished in the melt. The oxygen partial pressure is adjusted suchthat oxygen liberation from the fining agents (for example As, Sb or Sn)is still possible at the fining temperature, but the reduction of (lessnoble) glass components is effectively suppressed.

Attention should only be paid to the fact that minimum bubble size ofthe bubbling gas during this bubbling step has to be chosen sufficientlylarge (>0.5 mm) so that the bubbles ascend completely in the melt volumeand the fining process is not compromised by the entry of small bubblesinto the melt volume.

Afterwards, in the fining process happening in the hotter zone of thefining vessel, the chemical fining of the melt takes place by apreferably selective thermal reduction of the fining agent(s), theliberation of oxygen connected thereto and, simultaneously, the physicalfining through decrease of the viscosity in the melt.

Preferably the process is conducted so that the heating of the meltstream to the fining temperature is done impulsively and quickly. Forthat purpose the geometry of the fining zone is selected and the finingzone is provided with melt stream affecting fixtures so that a narrowdwell time spectrum is achieved in the melt volume and the heat inputinto the melt volume takes place efficiently.

Because of their form the fixtures can force a glass melt stream upwardsin the melt and thus assist the ascent of the bubbles. Preferred formsof such fixtures are slit skull segments, cooled by cooling agent. Thefixtures may likewise consist of cooled or un-cooled not reducingceramics and/or noble metals.

After the melt has passed through the hottest zone of the fining vesseland the actual fining is finished, it enters colder zones. In thesecolder zones on the one hand the adjustment of the exit temperature fromthe skull system takes place, which is below the temperature at whichthe corrosion of the following noble metal system begins. If one staysbelow a distinct temperature dependent on the matching of noble metalalloy and glass composition, no significant corrosion of the vessel wallby the melt takes place. On the other hand the re-sorption of thebubbles formed during the fining process, which are not large enough toascend and to exit the melting volume, takes place by reduced finingagent species.

The adjustment of the ranges of the different temperatures in skullsystems that are cooled with cooling agents takes place through acombination of constructive measures and process conduct.

The power input by the high-frequency radiation into the melt isdependent on the form and geometry of the high-frequency field. Thefield strength in the melt can be varied and adjusted by the distanceand the degree of coverage of the inductor and melt. Thereby the powerinput by the high-frequency radiation into the melt can be adjusted andvaried. This effect can be further strengthened or weakened by furtherconstructive measures like the positioning of the high-frequency shortcircuit and the thus occurring field displacement.

Extensive bubbling or agitating by a mechanical agitator cooled by acooling agent produces an intensive turbulent mixing of the melt, whichmakes the temperatures more uniform and warms zones that are otherwisecolder. Without intensive bubbling on the contrary a mostly laminar flowis formed with temperature layers and a stable temperature gradientbetween intensively cooled interfaces and the hot core zone of the meltor un-cooled interfaces, respectively. According to the high-frequencypower radiated into the melt and the adjusted melt throughput acombination of these effects leads to the desired temperature profile inthe entire melt system.

After the melt has passed through the colder zones of the fining vessel,it is withdrawn via a noble metal discharge system positioned in thewall or the bottom, which are both cooled by a cooling agent, and thenfed to further processing. This sort of withdrawal or discharge systemis disclosed in DE 103 29 718.9-45 and can be cooled or not cooled.

Further processing preferably utilizes a homogenization unit and acooling duct made of a noble metal alloy or of glass. In thehomogenization unit, made of silica glass, noble metal or a noble metalalloy, too, active (by means of an agitator) or passive (by fixtures)intensive mixing of the melt takes place, in order to adjust therefractive index uniformity and the striae quality to the necessarylevel according to the particular requirements. The agitator and thefixtures may consist of noble metals, noble metal alloys, silica glassand/or ceramics.

The cooling duct, as well as the homogenization unit, is directly orindirectly heatable. The cooling duct can at least in part be activelycooled.

The length and geometry of the cooling duct depends on the initialtemperature and the final temperature of the melt to be achieved.

In the homogenization unit a direction change of the melt from thehorizontal flow direction to a vertical downwards flow direction cantake place through a withdrawal opening in the bottom of the vessel. Inthis case a directly or indirectly heatable feeder system of one or moresegments that feeds the melt to the molding facilities is attached. Thisfeeder system consists of noble metal, a noble metal alloy, silica glassor ceramic. Length and diameter of the feeder segment(s) are fitted tothe desired feeding volume/time unit.

In the case of especially reduction-sensitive melts it can beadvantageous to conduct oxygen and/or ozone through the melt in thisalready relatively cold and correspondingly high viscosity area of themelt. The advantage of the gas input of oxidizing oxygen and/or ozone atsuch low temperatures is that the back reaction of the oxidized speciesto their corresponding reduced form is thermodynamically (because of thelow temperatures) and kinetically (because of the high viscosity)strongly suppressed. Hence, the transmission can again be amelioratedcritically (see below: melt example A).

In the case of glass for non-optical applications it is not important topay attention to remaining bubbles in the volume, instead oxygen and/orozone can be directed into the melt volume, for example, simply using abubbling lance made of noble metal.

In the case of glass for optical application the volume stream of thebubbling gas is preferably chosen to be smaller than 30 l/h. Nozzlediameter and tearing edge are to be adjusted to the gas throughput suchthat no turbulent streams are generated in the melt and no small oxygenbubbles (<0.5 mm) are formed that remain in the melt.

With the preferably used skull crucibles the metal pipes areshort-circuited in the area of the high-frequency coil. In a specialembodiment the metallic walls cooled by a cooling agent consist ofplatinum or a platinum alloy or of aluminium or an aluminium alloy. In afurther embodiment the metallic walls consist of copper, brass orINCONEL® and are coated by a layer of platinum, a platinum alloy orfluorine-containing synthetic material.

The addition of the glass batch of the melt is preferably performed incompacted form and during the melting process it is advantageous if theglass batch is mixed.

Preferred embodiments of the apparatus for performing the processaccording to the present invention include the melt crucible, the finingvessel and the homogenization unit with the features that are describedabove.

BRIEF DESCRIPTION OF THE DRAWING

The objects, features and advantages of the invention will now bedescribed in more detail with the aid of the following description ofthe preferred embodiments, with reference to the accompanying figures inwhich:

FIG. 1 is a schematic cross-sectional view of a first embodiment of asystem according to the invention for producing glass in which chemicalreduction of the components of the glass is avoided;

FIG. 2 is a schematic cross-sectional view of a second embodiment of asystem according to the invention for producing glass in which chemicalreduction of the components of the glass is avoided;

FIG. 3 is a schematic cross-sectional view of a third embodiment of asystem according to the invention for producing glass in which chemicalreduction of the components of the glass is avoided; and

FIG. 4 is a schematic cross-sectional view of a third embodiment of asystem according to the invention for producing glass in which chemicalreduction of the components of the glass is avoided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawing reference number 1 designates the melt and the drawingreference number 2 designates the glass level in each of theaccompanying figures. In the apparatus shown in FIG. 1 a melt crucible13, which comprises skull walls cooled by a cooling agent 3 and a cover6 made of flameproof material, is connected via a heatable connectionpiece 5 made of noble metal or silicate glass to a fining vessel 14,which likewise comprises skull walls cooled by a cooling agent 3 and acover 6 made of flameproof material. The melt crucible 13 and the finingvessel 14 are both encircled by inductors 4 and comprise bubblingnozzles 11 in a bottom area.

The fining vessel 14 comprises a glass melt flow influencing skull wall12 in its interior and is connected via a cooling duct 7 made of noblemetal or silicate glass to a homogenization unit 8. The homogenizationunit 8 made of noble metal or silicate glass comprises a cover 6 made offlameproof material, an agitator 9 made of noble metal or silicate glassand a heatable feeder device 10 made of noble metal or silicate glass.

The apparatus shown in FIG. 2 is different from that shown in FIG. 1 insuch a way that the melt crucible 13 and the fining vessel 14 areconnected to each other via a skull segment 5 a. Also homogenisationunit 8 in the embodiment of FIG. 2 comprises a bubbling nozzle 11.

The apparatus shown in FIG. 3 has a further modification in thehomogenization unit 8, which is a static homogenization unit 9 a insteadof an agitator. Furthermore the cooling duct and homogenization unit areformed in one piece.

In contrast the cooling duct and the homogenisation unit are not in onepiece in the embodiment shown in FIG. 4. The embodiment of the systemshown in FIG. 4 has as a connection between the homogenization system 8and the fining vessel 14 that comprises cooling duct 7, which is made ofnoble metal or silicate glass.

Instead of the noble metal a noble metal alloy can also be used in everyembodiment of the present invention. Instead of the bubbling nozzles 11in every case foamed, porous or perforated cooled structures can beused, too.

The glass that can be produced using the method according to the presentinvention preferably comprises the components shown in table I.

TABLE I Components for Glass Production Components Wt.-% P₂O₅, B₂O₃,SiO₂, F * 0-50 Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, PbO 0-80 WO₃, MoO₃ 0-30 GeO₂0-20 MgO, CaO, SrO, BaO 0-40 Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O 0-12 ZnO, TiO₂0-8  Σ Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, PbO 20-80  Σ WO₃, MoO₃, GeO₂ 0-40 Σalkali metal oxide 0-15 Σ alkaline earth metal oxide 0-30

Here and in the following tables and enumerations, the enumeration ofmultiple components means that these components can be incorporated intothe composition each independently in the indicated range.

The glasses that can be produced using the method according to thepresent invention particularly preferably comprise the compounds shownin table II.

TABLE II Components for Glass Production Components Wt.-% P₂O₅, B₂O₃,SiO₂, F 0-30 Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, PbO 0-60 WO₃, MoO₃ 0-30 GeO₂0-20 MgO, CaO, SrO, BaO 0-30 Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O 0-12 ZnO, TiO₂0-8  Σ Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, PbO 20-60  Σ WO₃, MoO₃, GeO₂ 0-40 Σalkali metal oxide 2-15 Σ alkaline earth metal oxide 0-30

The glasses that can be produced using the method according to thepresent invention exceptionally preferably comprise the compounds shownin table III.

TABLE III Components for Glass Production Components Wt.-% P₂O₅, B₂O₃,SiO₂, F 8-30 Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, PbO 10-50  WO₃, MoO₃ 0-30 GeO₂0-20 MgO, CaO, SrO, BaO 0-22 Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O 0-12 ZnO, TiO₂0-8  Σ Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, 20-60  PbO Σ WO₃, MoO₃, GeO₂ 0-40 Σalkali metal oxide 2-15 Σ alkaline earth metal oxide 0-30

The glasses that can be produced using the method according to thepresent invention most exceptionally preferably comprise the compoundsshown in table IV.

TABLE IV Preferably Components for Glass Production Components Wt.-%P₂O₅, B₂O₃, SiO₂, F 8-30 Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, PbO 10-50  WO₃,MoO₃ 0-16 GeO₂ 0-10 MgO, CaO, SrO, BaO 0-22 Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O0-12 ZnO, TiO₂ 0-8  Σ Nb₂O₅, Ta₂O₅, Bi₂O₃, Sb₂O₃, PbO 20-50  Σ WO₃,MoO₃, GeO₂ 0-20 Σ alkali metal oxide 2-15 Σ alkaline earth metal oxide0-20

Preferably the glasses have low contents of silicate and/or noblemetals, particularly preferred the glasses are free of silicate and/ornoble metals.

EXAMPLES

Examples 1 to 4 describe examples of glass produced according to thepresent invention and their properties (n_(d)=refractive index;v_(d)=Abbe number; ΔP_(g,F), τ_(i)=internal transmittance). Theinvention is not limited by these concrete examples.

Example 1

A glass of the following composition was produced:

P₂O₅: 21.0%

ΣNb₂O₅, Ta₂O₅, Sb₂O₃: 50.5%

ΣMgO, CaO, SrO, BaO: 19.0%

ΣLi₂O, Na₂O, K₂O, Rb₂O, Cs₂O: 4.5%

ΣZnO, TiO₂: 5.0%

A melting skull crucible and a fining skull crucible consisting ofAlMgSi1, connection segment, homogenization unit, agitator and feederconsisted of PtIr1 were used in the process.

The following melting parameters were used:

Melting: 1200° C.-1210° C., O₂-Bubbling: 3×50 l/h

Fining: 1220° C.-1230° C., O₂-Bubbling: 2×20 l/h

Mixing: 1180° C.

Feeder: 1150° C.

Ingots were produced.

The following optical values were measured:

n_(d)=1.92773;

v_(d)=20.61;

ΔP_(g,F)=−0.0312

τ_(i) (400 nm; 25 mm)=0.104; 0.002¹; 0.115²

τ_(i) (420 nm; 25 mm)=0.435; 0.232¹; 0.495²

τ_(i) (460 nm; 25 mm)=0.812; 0.748¹; 0.846²

τ_(i) (500 nm; 25 mm) 0.898; 0.858¹; 0.932²

The given reference values (¹) have been measured in a glass of the samecomposition that has been melted in a melting skull crucible made ofAlMgSi1 at 1210° C. and fined in a conventional fining chamber made ofPtIr1 at 1230° C. The cooling duct, the homogenization unit, theagitator and the feeder consisted of PtIr1. The reference melt was notbubbled with oxygen.

The given values (²) were achieved by additional O₂-bubbling through themelt at in other respects identical melting conditions as above. Forthat purpose the melt was bubbled with oxygen in a mixing crucible with1×15 l/h at 1175 to 1180° C.

Example 2

A glass of the following composition was produced:

P₂O₅: 22.8%

ΣNb₂O₅, Ta₂O₅, Sb₂O₃: 47.0%

ΣMoO₃, WO₃: 14.0%

ΣMgO, CaO, SrO, BaO: 2.0%

ΣLi₂O, Na₂O, K₂O, Rb₂O, Cs₂O: 9.2%

ΣTiO₂, GeO₂: 5.0%

A melting skull crucible and a fining skull crucible consisting ofAlMgSi1, connection segment, cooling duct, homogenization unit, agitatorand feeder consisted of PtIr1 were employed in the process.

The following melting parameters were adjusted:

Melting: 1110° C.-1120° C., O₂-Bubbling: 3×30 l/h

Fining: 1130° C.-1150° C., O₂-Bubbling: 2×150 l/h

Mixing: 1110-1120° C.

Feeder: 1100° C.

Ingots were produced.

The following optical values were measured:

n_(d)=1.97242;

v_(d)=22.65;

ΔP_(g,F)=0.0223

τ_(i) (400 nm; 25 mm)=0.070 (0.06)

τ_(i) (420 nm; 25 mm)=0.423 (0.36)

τ_(i) (500 nm; 25 mm)=0.875 (0.668)

The reference values given in brackets have been measured in a glass ofthe same composition that has been melted in a melting skull cruciblemade of AlMgSi1 at 1120° C. and fined in a conventional fining chambermade of PtIr1 at 1150° C. The cooling duct, the homogenization unit, theagitator and the feeder consisted of PtIr1. The reference melt was notbubbled with 3×30 l/h of oxygen in the melting skull.

Example 3

A glass for optical and electronic purposes of the following compositionwas produced:

ΣB₂O₃, SiO₂: 10.5%

ΣSb₂O₃, Bi₂O₃: 77.0%

ΣMgO, CaO, ZnO: 12.5%

A melting skull crucible, connection segment, cooling duct and feederconsisting of PtIr1 were used.

The following melting parameters were adjusted:

Melting: 1000° C.-1050° C., O₂-Bubbling: 3×50 l/h

Cooling track: 900-950° C.

Feeder: 850° C.

Glass flakes were produced.

The following optical values were measured:

n_(d)=2.101

Example 4

An optical glass of the following composition was produced:

SiO₂: 29.0%

ΣPbO, Sb₂O₃: 64.3%

ΣMgO, CaO, SrO, BaO: 2.0%

ΣLi₂O, Na₂O, K₂O, Rb₂O, Cs₂O: 6.7%

A melting skull crucible and a fining skull crucible consisting ofAlMgSi1, connection segment, cooling track, homogenization unit,agitator and feeder consisted of PtIr1 were used.

The following melting parameters were adjusted:

Melting: 1200° C.-1210° C., O₂-Bubbling: 3×30 l/h

Fining: 1275° C., O₂-Bubbling: 2×20 l/h

Mixing: 1180-1190° C.

Feeder: 1150° C.

Ingots were produced.

The following optical values were measured:

n_(d)=1.75815;

v_(d)=26.64;

ΔP_(g,F)=0.6067

τ_(i) (400 nm; 25 mm)=0.984

The above-mentioned glass is essentially free of other components. Thisis supposed to mean in the sense of the present invention that furthercomponents are not added to the glass and, if present at all, arepresent in the form and amount of impurities.

PARTS LIST

-   1—glass melt-   2—melt surface-   3—skull walls cooled by cooling agent-   4—inductor-   5—heatable connection piece made of noble metal or silicate glass-   5 a—skull connection element-   6—cover made of flameproof material-   7—heatable or coolable cooling duct made of noble metal or silicate    glass-   8—heatable homogenization unit made of noble metal or silicate glass-   9—agitator made of noble metal or silicate glass-   9 a—static homogenization unit made of noble metal-   10—heatable feeder device made of noble metal or silicate glass-   11—bubbling nozzles-   12—skull wall cooled by cooling agent (influencing the flow)-   13—melt crucible-   14—fining vessel

1. An apparatus for performing a process of producing glass so thatreduction of reduction-sensitive components is avoided or reduced, saidapparatus comprising a metallic skull crucible comprising a meltcrucible (13) for a glass melt and a fining vessel (14) connected to themelt crucible (13) to receive the glass melt from the melt crucible (13)when the process of producing glass is performed; wherein the meltcrucible and the fining vessel each comprise walls which are cooled by acooling agent and which are provided with high-frequency permeableslits; means for heating the glass melt in the melt crucible and theglass melt in the fining vessel inductively with a high-frequencyelectromagnetic alternating field; means for conducting at least oneoxidizing agent into the fining vessel, so that the at least oneoxidizing agent is introduced into the glass melt in the fining vesselwhen the process of producing glass is performed; and means forconducting at least one oxidizing substance into the melt crucible sothat said at least one oxidizing substance is introduced into the glassmelt in the melt crucible when the process of producing glass isperformed.
 2. The apparatus according to claim 1, wherein said means forconducting said at least one oxidizing agent into the fining vesselcomprises at least one bubbling nozzle (11).
 3. The apparatus accordingto claim 1, wherein said means for conducting said at least oneoxidizing agent into the fining vessel comprises foamed, porous orperforated cooled structures.
 4. The apparatus according to claim 1,wherein said high-frequency permeable slits have respective slit widthsof from 1.5 mm up to and including 4.0 mm.
 5. The apparatus according toclaim 4, wherein the slit widths are from 2.0 mm up to and including 3.0mm.
 6. The apparatus according to claim 1, wherein the melt cruciblecomprises coated or uncoated metal or the melt crucible comprises acoated or uncoated metal alloy.
 7. The apparatus according to claim 6,wherein the metal or the metal alloy is aluminium alloy, a nickel basisalloy, copper, brass, a noble metal or steel.
 8. The apparatus accordingto claim 6, wherein the metal or the metal alloy is coated with asynthetic material containing fluorine.
 9. The apparatus according toclaim 1, further comprising a burner supplied with fossil fuels toinitiate a glass melting process to form the glass melt.
 10. Theapparatus according to claim 1, wherein the melt crucible (13) isconnected with the fining vessel (14) via a heatable platinum orsilicate connection piece (5) through which the glass melt istransferred from the melt crucible into the fining vessel.
 11. Theapparatus according to claim 1, wherein the melt crucible (13) isconnected with the fining vessel (14) via a skull connection element (5a) through which the glass melt is transferred from the melt crucibleinto the fining vessel.
 12. The apparatus according to claim 1, furthercomprising a homogenization unit (8) connected to the fining vessel(14).
 13. The apparatus according to claim 12, wherein thehomogenization unit (8) comprises an agitator (9) made of noble metal orsilicate glass.
 14. The apparatus according to claim 12, wherein thehomogenization unit (8) is a static homogenization unit made of noblemetal.